Fabrication of high-performance noble-metal-free bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water is a promising strategy toward future carbon-neutral economy. Herein, a one-pot hydrothermal synthesis of cobalt sulfide/nickel sulfide heterostructure supported by nickel foam (CoS /NiS@NF) was performed. The Ni foam acted as the three-dimensional conducting substrate as well as the source of nickel for NiS. The formation of CoS /NiS@NF was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The formation of CoS /NiS@NF facilitated easy charge transport and showed synergistic electrocatalytic effect toward HER, OER, and overall water splitting in alkaline medium. Remarkably, CoS /NiS@NF showed catalytic activity comparable with that of benchmarking electrocatalysts Pt/C and RuO. For CoS /NiS@NF, overpotentials of 204 and 280 mV were required to achieve current densities of 10 and 20 mA cm for HER and OER, respectively, in 1.0 M KOH solution. A two-electrode system was formulated for overall water splitting reaction, which showed current densities of 10 and 50 mA cm at 1.572 and 1.684 V, respectively. The prepared catalyst exhibited excellent durability in HER and OER catalyzing conditions and also in overall water splitting operation. Therefore, CoS /NiS@NF could be a promising noble-metal-free electrocatalyst for overall water splitting application.

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